Heretofore, a water-based ink for ink-jet recording comprising a pigment and water has been used widely. When recording is performed with the water-based ink, permeability of the water-based ink relative to a recording medium affects greatly a drying ability of a recorded object. For the improvement of the drying ability of the recorded object, a surfactant or a penetrant may be added to the water-based ink to increase the permeability thereof relative to a recording medium by reducing surface tension of the water-based ink. However, when the permeability of the water-based ink relative to the recording medium is increased by reducing the surface tension of the water-based ink, an optical density (OD) value of the recorded object is decreased and printing quality may be decreased in this point.
As a method for increasing the optical density (OD) value of the recorded object, there is a method of preliminarily applying multivalent metal salt on the recording medium. According to such a method, since the pigment is aggregated very fast on the surface of the recording medium, the optical density (OD) value of the recorded object is increased. However, it takes time to apply the multivalent metal salt on the recording medium.
On the other hand, when the multivalent metal salt is directly added to the water-based ink, the time for preliminarily applying the multivalent metal salt is not required. However, in such a method, the pigment is aggregated in the water-based ink before recording, and unstable ejection of ink such as unejection or ejection deflection may occur at the time of ejecting the water-based ink from a nozzle.